Generally, a fuel cell is an electric power generating system that converts chemical reaction energy between hydrogen and oxygen contained in a hydrocarbon-based fuel such as methanol and natural gas, directly into electrical energy. The fuel cell can use the electricity generated from the chemical reaction between hydrogen and oxygen without a combustion process or heat, which is a by-product of the reaction.
Fuel cells are classified into various groups including a Phosphoric Acid Fuel Cell (PAFC) that operates at around 150 to 200° C., a Molten Carbonate Fuel Cell (MCFC) that operates at a high temperature of between 600 and 700° C., a Solid Oxide Fuel Cell (SOFC) that operates at a very high temperature of over 1000° C., and a Polymer Electrolyte Membrane Fuel Cell (PEMFC) and an Alkaline Fuel Cell (AFC) that operate at between room temperature and a temperature no higher than 100° C. These fuel cells all operate on the same fundamental principles, but the types of fuel, operating temperature, catalyst, and electrolyte that are used differ from each other.
The recently developed PEMFC has an excellent output characteristic and fast starting and response characteristics as well as a low operating temperature, when compared to other types of fuel cells. It also has an advantage in that it can be applied to a wide range of applications such as a distributed power source for houses and public buildings, as a small power source for electronic devices, and as a mobile power source for a car. The PEMFC uses hydrogen obtained by reforming methanol, ethanol, or natural gas as a fuel.
The basic structure of a system for a PEMFC comprises a fuel cell body called a stack, as well as a fuel tank and a fuel pump that supplies fuel from the fuel tank to the stack. It further requires a reformer that generates converts the fuel into hydrogen while supplying the fuel stored in the fuel tank to the stack.
The PEMFC generates electricity by supplying the fuel stored in the fuel tank to the reformer using the fuel pump, generating hydrogen gas in the reformer, and reacting the hydrogen gas with oxygen in the stack.
In addition, the fuel cell may have a Direct Oxidation Fuel Cell (DOFC) scheme, such as a Direct Methanol Fuel Cell (DMFC) that can directly supply liquid methanol fuel to the stack. Unlike the PEMFC, the fuel cell of the DOFC scheme does not require the reformer.
FIG. 5 is a cross-sectional view that shows a stack that is used in a conventional fuel cell system. As shown, the stack 10, which generates the electricity in the fuel cell, includes numerous unit cells that are stacked with each other. Each unit cell comprises a membrane-electrode assembly (MEA) 11 and separators 13, 13′, which are also referred to as bipolar plates.
The MEA 11 comprises an anode and a cathode that are each positioned on a side with an electrolyte membrane interposed therebetween. The separators 13, 13′ form a fuel path 15 and an oxidizer path 17 on the surfaces where they are attached to the anode and cathode, respectively. Thus, the separators 13 supply hydrogen gas to the anode through the fuel path 15 and the separators 13′ supply air containing oxygen to the cathode through the oxidizer path 17.
As a result, the hydrogen gas is oxidized at the anode and the oxygen is reduced at the cathode. The protons that are generated from the hydrogen oxidation reaction move to the cathode through the electrolyte membrane while electrons move to the cathode through an external wire to thereby generate electricity. Water is generated as a byproduct from the reduction reaction of protons, electrons, and oxygen at the cathode.
The stack 10 of the conventional fuel cell system is configured to expose unit cells in the region where the stack 10 is set up. This may cause water to condense on the surface of the unit cells, that is, on the external surface of the separators 13, 13′, due to a temperature gradient in the space. If a conductive material contacts the external surface of the separators, an electrical short may occur. In other words, although the separators 13, 13′ on both sides of the membrane-electrode assembly 11 are supposed to be insulated, a short may occur between the separators 13, 13′ due to the condensed water and the conductive material. At the very least, such a short circuit may block electricity from efficiently reaching the load and may cause damage to the entire fuel cell system. This may also result in a more harmful situation such as a fire or a fatal accident.